Free view television consists in transmitting several views of the same scene, captured with different cameras. The user is free to display any of the transmitted views of the scene, or even to synthesize an intermediate view from transmitted views, which corresponds to the view of a virtual camera. Therefore real views are completed at the reception side, by so called synthetic or virtual views. The user selects freely the viewing position and the angle of the virtual camera.
Besides, 3D television also relies on multi view video. In stereo 3D, both a right and a left view are displayed so that the user enjoys 3D images. A baseline correction on a right view or a left view is sometimes needed, depending on the user environment. For instance, it depends on the dimensions of the display, on sitting distance. In this case, a left or right virtual view is synthesized.
As for standard TV, copyrights protection remains a concern in free view video. Among many alternative copyrights managements systems, watermarking techniques embed imperceptible information hidden in images. This information is used in forensics to identify the source of an illegal copy. However watermark embedding and watermark detection in multi view video are more complicated than in mono view video. Indeed, the watermark must be detectable on any real or virtual view displayed by the TV set. Therefore the watermark needs to be embedded in a coherent ways in each view so that it appears correctly in any real or virtual view.
A main challenge has emerged as the coherent watermark embedding allowing known watermark detection techniques in virtual views rendered for arbitrary cameras.
In “Watermarking of free-view vided” (in “IEEE Transactions on Image Processing” volume 19, pages 1785-1797, July 2010), A. Koz, C. Cigla and A. Alatan disclose a method for embedding a watermark into multi views by exploiting the spatial masking properties of the human visual system. They also disclose a method for detecting watermark by exploiting the position and the rotation of a virtual camera. However the method for watermark detection requires at least one of the original views and the parameters of cameras, which are not always available. In case of unknown cameras parameters, they disclose to use the original views, along with corresponding depth-map information, to estimate the camera position and orientation of the synthesized view. The method comprises a step of transforming the original video with respect to the estimated parameters, and a step of subtracting it to the synthesized view. The correlation between the resulting signal and the watermark signal provides better performance in the watermark detection. However the estimation of the cameras parameters requires heavy processing, such watermark detection is complex and time consuming. Besides, the detection performances are sensitive to the cameras parameters estimation.
Thus known methods for watermark detection in free view video raise the issue of the detection for rendered views both for known and unknown cameras position and orientation. As a consequence, the issue of a coherent watermark embedding enhancing watermark detection is raised. In the state of the art, these issues are solved using one of the original views.
A method for watermark embedding in multi view video allowing blind detection in any rendered view, real or virtual, is therefore is needed.